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• Coal beds in the XPA are contained in the lower part of the Williams Fork Formation member of the Upper Cretaceous Mesaverde Group. The coal-bearing zone, which is approximately 650 ft thick in the XPA, rests immediately on top of the Rollins Sandstone member of the Iles Formation, the lower-most portion of the Mesaverde Group present in the North Fork Valley area. The Rollins Sandstone is amixed-environment unit, representing barrier island and beach sands in many places, and distinctly delta-front sands, distributary mouth bars, channel fill sands, and so on, in others. The coal-bearing part of the Williams Fork has been divided throughout the southern Piceance basin into the lower Bowie Shale and upper Paonia Shale. The Bowie Shale consists essentially of the coal-bearing interval from the top of the Rollins Sandstone to the top of the "D" or middle sandstone, which is overlain by the D series of coal beds in the Terror Creek area. The Bowie Shale as so described consists of mostly shaley claystones, mudstones, and siltstones, with a few thin shell-rich limestones and lenticular sandstones of predominantly marine and brackish water origin, and of several coal beds and immediately adjacent sediments which reflect fresh-water environments. The Paonia Shale consists of the coal bearing interval above the D (middle) sandstone, and contains generally more coarse-grained clastics of brackish and fresh water origin than the Bowie, although significant marine units are still present (for example the "upper sandstone" in Coai Basin to the northeast.) Coal has been mined in the Somerset -Paonia area since before 1900. Until the • opening of the Orchard Valley Mine directly north of Paonia in 1976, most production was centered around the mining camps of Somerset and Bowie, where the nomenclature of the Somerset coal field was developed. The general sequence in the Bowie area according to previous work is: E, or Hawksnest 4 to 12 ft thick; mined at Hawks Nest Mines, Blue Ribbon and Mt. Gunnison Interval, 200-300 ft D, or Oliver 1 to 3 beds, 0 to10 ft thick, up to 25 ft thick where one or more beds combine; mined at Oliver Mines. Interval, 100-200 it C, or Bear 4 to 8 ft thick; mined at Clark, Bear, and Somerset Interval, 20-80 ft Upper B, B-2, or Upper Somerset 6 to 15 ft thick, mined at King (old Bowie), Somerset, Bear, Sanborn Creek, and Mt. Gunnison • Interval, 0-20 ft